1. Scheduling (Priority Based)
Lecture 09

2. CPU Scheduling Scheduling processes in the ready queue
Short-term scheduler
Different types of schedulers
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3. Life of a Process
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4. Histogram of CPU-burst Times
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5. CPU Scheduler Short-term scheduler
Selects a process from among the processes in the ready queue
Invokes the dispatcher to have the CPU allocated to the selected process
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6. Dispatcher
Dispatcher gives control of the CPU to the process selected by the short-term scheduler; this involves:
switching context
switching to user mode
jumping to the proper location in the user program to start (or restart) it
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7. Dispatcher
Dispatch latency – time it takes for the dispatcher to stop one process and start another running.
Typically, a few microseconds
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8. CPU Scheduler CPU scheduling decisions may take place when a process:
Switches from running to waiting state
Switches from running to ready state
Switches from waiting to ready
Terminates
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9. CPU Scheduler Scheduling under 1 and 4 is nonpreemptive.
All other scheduling is preemptive.
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10. Scheduling Criteria CPU utilization – keep the CPU as busy as possible
Throughput – # of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process
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11. Scheduling Criteria
Waiting time – amount of time a process has been waiting in the ready queue
Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
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12. Optimization Criteria
Maximize CPU utilization
Maximize throughput
Minimize turnaround time
Minimize waiting time
Minimize response time
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13. FCFS Scheduling
The process that enters the ready queue first is scheduled first, regardless of the size of its next CPU burst
Example: Process Burst Time P P P
Suppose that processes arrive into the system in the order: P1, P2 , P3
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14. FCFS Scheduling Processes are served in the order: P1, P2, P3
The Gantt Chart for the schedule is:
Waiting times P1 = 0; P2 = 24; P3 = 27
Average waiting time: ( )/3 = 17 P1 P2 P3 24 27 30
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15. FCFS Scheduling
Suppose that processes arrive in the order: P2 , P3 , P1 .
The Gantt chart for the schedule is:
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: ( )/3 = 3
Convoy effect short process behind long process P1 P3 P2 6 3 30
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16. Shortest-Job-First (SJF) Scheduling
Process with the shortest CPU burst is scheduled first.
Non-preemptive – once CPU given to a process it cannot be preempted until completes its CPU burst.
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17. Shortest-Job-First (SJF) Scheduling
Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt it—Shortest-Remaining-Time-First (SRTF).
SJF is optimal non-preemptive scheduling algorithm – gives minimum average waiting time for a given set of processes.
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18. Non-Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4
Gantt chart
Average waiting time = ( )/4 = 4 P1 P3 P2 7 16 P4 12
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19. Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4
Gantt chart
Average waiting time = ( )/4 = 3 P3 P2 4 2 11 P4 5 7 P1 16
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20. Example of Shortest-remaining-time-first
Now we add the concepts of varying arrival times and preemption to the analysis
ProcessA arri Arrival TimeT Burst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
Preemptive SJF Gantt Chart
Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec
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21. SJF—CPU Burst of a Process
Can only estimate the length of the next CPU burst.
Can be done by using the length of previous CPU bursts, using exponential averaging.
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22. SJF is Optimal
Logical Argument: Decrease in the wait times for short processes is much more than increase in the wait times for long processes P1 P2 P3 P3 P2 P1
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23. Priority Scheduling
A priority number (integer) is associated with each process
The CPU is allocated to the process with the highest priority (smallest integer  highest priority).
Preemptive
Non-preemptive
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24. Priority Scheduling
SJF is a priority scheduling where priority is the predicted next CPU burst time.
Problem  Starvation – low priority processes may never execute.
Solution  Aging – as time progresses increase the priority of the process.
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25. Example of Priority Scheduling
Process A arrive Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
Average waiting time = 8.2 msec
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26. Round Robin (RR)
Each process gets a small unit of CPU time, called time slice or quantum, which is usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.
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27. Round Robin (RR)
If there are n processes in the ready queue, the time quantum is q, and context switch time is tcs, then no process waits more than (n-1)(q+tcs) time units
Used in time-sharing systems where response time is an important performance criteria
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28. Round Robin (RR) Performance q large  FCFS
q small  q must be large with respect to context switch, otherwise overhead is too high.
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29. Round Robin Example The Gantt chart with quantum 20 is:
Process Burst Time
P1 53 — 33 — 13
P2 17
P3 68 — 48 — 28 — 8
P — 4
The Gantt chart with quantum 20 is: P1 P2 P3 P4 20 37 57 77 97
117
121
134
154
162
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30. Round Robin Example Average waiting time = 73
Process Turnaround Time Waiting Time
P – 53 = 81
P – 17 = 20
P – 68 = 94
P – 24 = 97
Average waiting time = 73
Average waiting time for SJF = 38
Typically, higher average turnaround than SJF, but better response.
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31. Example of RR with Time Quantum = 4
Process Burst Time
P1 24
P2 3
P3 3
The Gantt chart is:
Typically, higher average turnaround than SJF, but better response
q should be large compared to context switch time
q usually 10ms to 100ms, context switch < 10 usec
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32. Quantum vs Context Switch
Process Time = 10
Quantum Context
Switches
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33. Turnaround Time vs Quantum
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